1. Field of the Invention
This device relates to end effectors manipulated by robots, for positioning, and more particularly to centering a channel-shaped workpiece under the end effector.
2. Background Information
A need has developed in industry for an end effector that can be picked up and manipulated by a robot and used to position a workpiece and then drill a hole in the workpiece that can be used for subsequent assembly of the workpiece with other parts of a product. An end effector with this capability would be useful in many traditional manufacturing applications, but finds especially significant utility in a virtual tooling assembly, such as that which is disclosed in U.S. Pat. No. 5,560,102 to Micale et al entitled "PANEL AND FUSELAGE ASSEMBLY," that is assigned to the assignee of this invention and is incorporated herein by reference.
In traditional manufacturing, parts are normally held in tooling fixtures and assembly jigs is to accurately locate the parts by butting their edges against hard stops on the tooling. From those stops, locations on the part are determined for holes, fasteners, and other manufacturing processes which are performed on the part. Assuming that the part is located properly on the tooling and that the drill guides and other parts of the tooling for performing the manufacturing processes have been accurately located, the part will be accurately manufactured. However, in the real-life environment of a factory, the tooling receives some heavy and sometimes abusive use and the accuracy of the stops, drill guides, and other elements of the tooling frequently become worn or otherwise out of adjustment. When this happens, the manufacturing processes on the part can be performed at an incorrect location on the part resulting in slight misalignment of the elements of the manufactured product, or if the misalignment is severe, a rejected part. The result is increased cost of manufacturing and a degradation of quality.
A new manufacturing technique has been developed called "soft tooling" or "virtual tooling" which eliminates the need for hard tooling used for positioning of holes, fasteners, and other manufacturing processes. The soft tooling technique uses information from the original engineering drawings of the part, such as a panel, to locate the holes and other items on the panel without reference to hard tooling reference stops. One technique is to mount the panel on a fixture which holds it in a fixed position and contour but without reliance on the position of any hard stops on the fixture. The panel is then machined with a router tool around its entire edge to give it the precise exterior dimensions required by the original engineering data. The holes, fasteners, and parts which are to be placed at particular positions on the panel, are located by positioning or coordination holes drilled in the panel by an end effector held in a highly accurate robot. The positioning of the robot is accomplished by software using positioning information derived from the engineering drawing of that part. Any other components which must be attached to the panel are drilled at the same time or on another fixture so that the positioning holes in the panel and on the other components are precisely accurate, well within the required tolerance. After drilling the positioning holes, the panel and other components are removed from the fixture, and the components may be drilled and assembled to the part using the positioning holes. In this way, the components are always located accurately, and the care and recalibration of hard tooling becomes an obsolete requirement.
One such end effector is disclosed in U.S. Pat. No. 5,127,139 entitled "STRINGER CLIP END EFFECTOR" by McCowin, et al, which is assigned to the assignee of this invention. Another end effector desired for aircraft manufacturing is one which will align a channel-shaped workpiece such as a stringer at a desired position with respect to the drill axis and hold the stringer in that position while it drills a hole in a stringer channel at the desired lateral position and at the desired position along its length. This end effector is disclosed in U.S. Pat. No. 5,299,894 to McCowin, which is assigned to the assignee of this invention.
The robot used in the particular embodiment of the "virtual tooling" or soft tooling development noted above is a JOMACH 16, manufactured by JOBS Corp. of Italy, which is most accurate when using a tapered shank coupling for the end effector. This coupling does not include provision for auxiliary power for the end effector, so all of the functions must be powered by the rotational power delivered by the robot's spindle, or the force that the robot can exert in the "Z" direction. Since one of the functions required by the drilling and centering end effector is to position the center of the stringer channel under the drill so that the hole is drilled in the exact center of the stringer channel, or at the desired position in the channel specified by the engineering data, the lateral movement of the stringer must be accomplished using rotational power or "Z" axis pushing force.
The solution to this centering problem disclosed in U.S. Pat. No. 5,299,894, has a pair of cam rollers projecting down from a base plate equidistant from the centerline of the end effector and aligned on the lateral axis that intersects the centerline of the end effector to center a channel-shaped workpiece, such as a stringer, on the centerline of the end effector. The cam rollers work by entering into the channel of the workpiece, which may be slightly misaligned with the centerline and then rotating about the centerline of the cam roller shaft. Rotation of the shaft engages one of a pair of rollers with the side of the channel and pushes the stringer toward a centered position. Once the stringer reaches the centered position both rollers are engaged with the sides of the channel in the stringer and the stringer is centered on the lateral axis connecting the two cam roller shafts of the two cam rollers, and a hole is drilled in the center of the channel of the stringer by a drill.
The problem with the solution in the U.S. Pat. No. 5,299,894 is that it is only effective if the stringer is straight and flat. If not, the center of the drill end effector, will not correspond to the desired centered location on the stringer. This is because the center point is being determined using typically four points in contact with the walls of the channel in the workpiece and two points in contact with the bottom of the channel in the workpiece. If the workpiece is curved the center of the drill will not correspond to the desired center of the stringer.